A coupler can be implemented as a power coupler or a directional coupler. Power couplers (e.g., power splitters and, when used in reverse, power combiners) and directional couplers are passive devices that can be used in the field of radio technology. A coupler can be implemented as a pair of transmission lines that provide mutual inductance and capacitance. A power coupler can couple a defined amount of the electromagnetic power in a transmission line to another port where the power can be used in another circuit. A directional coupler can be configured to couple power flowing in one direction such that power entering the output port is not coupled. Directional couplers and power dividers have many applications, these include; providing a signal sample for measurement or monitoring, feedback, combining feeds to and from antennae, and providing taps for cable distributed systems such as cable TV.
A coupler can have a coupling factor that defines an output power at an output port of a coupler relative to a power provided at an input port of the coupler. The coupling factor represents a primary property of a directional coupler. Coupling factor is a negative quantity when expressed in decibel ratio, although in some examples the minus sign is dropped (but still implied). The coupling factor can vary with frequency.